Polystyrene is a widely used commercially available plastic. Pure polystyrene is a hard plastic having limited flexibility and is thus well suited to applications where its level of stiffness is acceptable. In other situations, the limited flexibility or rigidity of pure polystyrene may be unacceptable. In these situations, styrene-based copolymers, and particularly polystyrene resins that are modified with organic rubber particles or other elastomers, have been found useful. The rubber modified styrene compositions contain a matrix of a styrenic resin and a rubber component in particle form dispersed in the matrix. These rubber modified styrene compositions include compositions typically referred to as high impact polystyrene, or HIPS.
In the final HIPS product, the rubber particles are incorporated into the HIPS product, wherein rubber particles make up a rubber phase volume fraction (RPVF). The properties of HIPS may be affected by the RPVF. It has been observed that a larger RPVF may confer some desirable properties to HIPS. Many attempts have been made to increase the RPVF by increasing the size of polystyrene occlusions trapped within the rubber particles. While the size and distribution of the rubber particles are controlled by several factors including bulk viscosity of the two phase system, the shear rate applied to the system, the level of emulsifier, the surface tension of the solution and the viscosity ratio of the discontinuous phase, or rubber phase, to the viscosity of the matrix, controlling the size of the occlusions has been more elusive. It would therefore be desirable to produce a HIPS product having an increased RPVF. It would also be desirable to have a method that is able to control and/or increase the size of the polystyrene occlusions.